(1) Field of the Invention
The present invention relates to an apparatus and method for radio communication in a radio network, and particularly to such an apparatus and method applicable to a distributed environment, i.e. a network which has a distributed topology.
(2) Description of the Related Art
Radio networks with the use of radio wave or infrared light have been in great demand these days because they require no communication cables. Radio terminals can intercommunicate from any site within their communication ranges, and be used on the move when they are reduced to portable.
However, radio networks often suffer from so called "Hidden Terminal Problem" which does not affect wired networks and is caused by a radio terminal outside their communication ranges.
FIG. 1 is an illustration to be used to explain a Hidden Terminal Problem (hereinafter HTP).
A, B, and C are radio terminals, and the circles represent respective communication ranges. When terminal A is communicating with terminal B, terminal C is getting no signal from terminal A. Consequently, terminal C might start to communicate with terminal B, without knowing the communications between terminals A and B. If terminal B is thus communicated from both terminals A and C, the communication carried out so far between terminals A and B ends up in vain. Thus, the occurrence of HTP requires retransmission, reducing data transmission throughput in radio networks.
In order to solve the HTP, various radio network communication schemes have been proposed as follows.
&lt;CSMA/CA+Ack&gt;
A communication scheme called CSMA/CA+Ack (Carrier Sense Multiple Access with Collision Avoidance plus Acknowledgement) has been proposed by W. Diepstraten in "A Distributed Access Protocol Proposal Supporting Time Bounded Service" (IEEE Working Group Paper 802.11-93/70).
Although this scheme is not directly concerned with the solution of HTP, it is a basic communication scheme in networks and referred to later in the explanation of communication schemes.
Generally, multiple accesses in a network can be realized based on a MAC (Medium Access Control) layer protocol. The CSMA/CA+Ack is a MAC layer protocol based on CSMA/CA which is widely used as a MAC layer protocol in a radio network. The CSMA/CA+Ack has a function for acknowledgement and retransmission of frames at a MAC layer level, in addition to the functions of CSMA/CA. The CSMA/CA is more suitable to radio communications than CSMA/CD (CSMA with Collision Detection) which is widely used as the MAC layer protocol in cable networks.
According to the CSMA/CA+Ack, a first terminal which is ready to transmit data makes sure that the transmission path has no signal for a predetermined gap time period, and then starts the data transmission in frames to a second terminal. The second terminal, which has received the frames returns a confirming frame to the first terminal in order to report the successful reception of the frames, after having made sure that the transmission path has no signal for a predetermined gap time period.
Although the use of the confirming frames has improved the reliability of the data transmission, this scheme presupposes that a signal transmitted by any of terminals is distributed to all the terminals in the network, which does not insist on solving the HTP.
&lt;K. Biba&gt;
K. Biba has proposed a scheme of solving HTP in "A hybrid Wireless MAC Protocol Supporting Asynchronous and Synchronous MSDU Delivery Service" (IEEE Working Group Paper 802.11-91/92). In order to avoid HTP, a transmitting terminal and a receiving terminal establishes a connection every time a frame is transmitted, and informs the other terminals of a time period during which the transmission path is occupied in establishing a connection.
According to this scheme, a transmitting terminal sends an RTS (request to send) frame prior to a data transmission, and a receiving terminal returns a CTS (Clear to Send) frame to report the reception of the RTS frame. Then, the transmitting terminal sends data in frames, and the receiving terminal returns an ACK frame to report the receipt of the frames. An RTS frame carries the length of data to be transmitted, and a CTS frame carries the length of data to be received. Prior to the transmission of RTS frames and CTS frames, the condition of the transmission path is checked with CSMA. The terminals other than the destination of an RTS frame check the length of the data carried in the frame and refrain from accessing the transmission path until the transmission and acknowledgement of the data is completed. In the same manner, the terminals other than the destination of a CTS frame do not access the transmission path until the transmission and acknowledgement of the data is completed. This is how HTP is solved in this system.
However, a connection must be established per frame, so that the efficiency of data transmission is decreased when large data being divided into a plurality of frames are transmitted.
For another problem, the transmission of an RTS (or CTS) frame may be sometimes unsuccessful because of a collision. In that case, a terminal which successfully received the RTS (or CTS) frame must be deprived of the access to the transmission path in vain for the time period corresponding to the unexecuted data transmission.
For further another problem, there is a possibility that a terminal which moved into the transmission range of the receiving terminal starts another data transmission without knowing the preceding data transmission. Thus, HTP resulting from a terminal's movement cannot be solved.
&lt;Japanese Laid-open Patent Application No. 5-260051&gt;
The scheme proposed in Japanese Laid-open Patent Application No. 5-260051 is used in a system composed of a plurality of terminals, and a base station which communicates with all the terminals. An available frequency band is divided into a message channel, an up link channel, and a down link channel.
A terminal which is ready to transmit data checks the presence or absence of a channel tone, and in the case of its absence, transmits a channel tone on the up link channel. Detecting the channel tone, the base station transmits the same channel tone on the down link channel. Detecting the return of the channel tone, the terminal starts data transmission.
This scheme requires the base station to solve HTP, so that in case that the base station is out of order, all the terminals in the network becomes unable to communicate, deteriorating the reliability of the entire network.
&lt;U.S. Pat. No. 4,409,687&gt;
The communication scheme disclosed in U.S. Pat. No. 4,409,687 can use a plurality of channels assigned to different frequencies, and is used in a system composed of a plurality of terminals and a base station. A terminal in the transmission mode scans all the channels prior to a data transmission, and if a channel in the idle state is found, the terminal transmits a busy tone on the channel and further transmits a predetermined group tone. Receiving the busy tone from the terminal, the base station transmits the busy tone on the channel, and further repeats the group tone. A terminal in the reception mode scans all the channels, and establishes a connection with the transmitter in response to the reception of the predetermined group tone.
This scheme can solve HTP; however, it still requires a base station (repeater), and as a result this system suffers from the same problems as the above mentioned Japanese Laid-open Patent Application No. 5-260051.
&lt;U.S. Pat. Nos. 4,360,927 and 4,658,435&gt;
U.S. Pat. Nos. 4,360,927 and 4,658,435 have also disclose communication schemes capable of solving the HTP and of using a plurality of channels; however, these schemes require a base station, suffering the same problems as mentioned above.